Saccharification of cellulose

ABSTRACT

A method for effecting saccharification of raw cellulosic material taken from any available source. The cellulosic material is hydrolyzed by immersing it in a bath of anhydrous liquid hydrogen chloride to yield usable glucose and other products. The cellulosic material may be processed through one or more pre-treatment steps, each acting to enhance the yield of glucose and other products when the pre-treated material is subjected to hydrolyzation by anhydrous liquid hydrogen chloride. The raw material may first be immersed in a caustic solution which swells the cellular structure to render it more reactive and acts to solubilize the lignins and other compounds contained therein, these being washed away to provide a prepared cellulosic material. In another or second preparatory step, the material may be further prepared by subjecting it to a viscose process to produce a viscous solution of sodium cellulose xanthate in sodium hydroxide, from which solution the aqueous phase is separated, leaving a viscose intermediate. In the final step, the viscose intermediate is hydrolyzed by immersing it in a bath of anhydrous liquid hydrogen chloride.

RELATED APPLICATION

This application is a continuation-in-part of my copending applicationSer. No. 936,646, filed Aug. 24, 1978, of the same title.

BACKGROUND OF INVENTION

This invention relates generally to a method to effect saccharificationof cellulosic material, and more particularly to a method for theproduction of ethyl alcohol or ethanol from cellulosic waste materialsaccharified by an anhydrous liquefied hydrogen halide such as hydrogenchloride.

The hydrolysis of cellulose yields a mixture of simple reducing sugars,mainly glucose. These hydrolysis products are convertible byfermentation to ethyl alcohol which can be used as a liquid fuel toreplace gasoline. In terms of available energy, expressed either as theheat of combustion of cellulose or of the glucose or alcoholtheoretically obtainable therefrom, a pound of cellulose is equivalentto 0.35 lb. of gasoline.

Billions of tons of carbon are fixed every year on the land area of theearth by photosynthesis, out of which about half appears in the form ofcellulose. The sheer magnitude of this potential source and the factthat petroleum-derived fuels are becoming increasingly scarce and moreexpensive, has awakened great interest in the exploitation of otherwiseunused waste cellulosic materials.

About two hundred ninety million tons per year of residual,institutional and commercial solid wastes containing approximately fiftypercent paper and other cellulosic materials are generated in the UnitedStates. Moreover, about sixty million tons of bagasse are available.These altogether contain about one hundred and twenty million tons ofusable cellulose, the energy equivalence of which is nearly a fifth ofthe current U.S. gasoline consumption. Furthermore, 2.3 billion tons peryear of agricultural wastes in the United States having a high cellulosecontent, increase this supply significantly. Despite these many andvaried sources of available cellulose, a major industrial alcoholfermentation process can only be successful if the reducing sugars canbe derived from a cellulosic material at a sufficiently low price.

Early efforts in this direction were mainly geared to the acidhydrolysis of wood products to sugars. In recent years, various methodshave been proposed to carry out cellulose degradation through enzymaticmeans. Thus U.S. Pat. Nos. 3,642,580; 3,764,475 and 4,009,075 effect theconversion of cellulose to simple sugars by enzymatic hydrolysis, theresultant sugars then being fermented by years to produce alcohol.

Hydrolysis is a chemical reaction in which water reacts with a compoundto cause the decomposition or splitting thereof. Water in the form ofits hydrogen and hydroxyl ions adds to the cleaved compound. Hydrolysisis generally catalyzed by ions. In the absence of ions, hydrolysis maybe a very slow process. Thus the use of an acid as a catalyst increasesthe concentration of hydrogen or hydroxyl ions with a corresponding risein the rate of hydrolysis. The hydrolysis of some organic compounds isalso catalyzed by certain enzymes.

It is known that the polysaccharide components of wood and othercellulosic materials can be partially or completely hydrolyzed by acidcatalyzed reactions in which the cellulose is converted to glucose andthe hemicellulose mostly to xylose. The resultant syrupy mixture ormolasses of wood sugar may be used as cattle feed, or it may befermented to alcohol. Alternatively, the glucose derived in this mannermay be separated from the wood sugars and purified.

Cellulose is a D-glucose polymer with β (1-4) linkages. Relatedcompounds are polymers of D-xylose with β (1-4)linkages and side chainsof arabinose and other sugars. The glycosidic bonds between thesevarious sugar sub-units --0-- are akin to ether bonds, acetal bonds andhemiacetal bonds, which bonds are hydrolyzable by acids. An aqueous acidnot only acts as a catalyst for these reactions, but also supplies waterthereto. This characteristic of an aqueous acid gives rise to a majordrawback in the hydrolysis of cellulose.

When, for example, cellulosic material is treated with an aqueoussolution of hydrogen chloride (hydrochloric acid) as a catalystpromoting hydrolysis, those cellulose molecules having exposed glucosesub-units will be hydrolyzed. But the free glucose resulting from thisprocess is hydroscopic in nature and renders the remaining celluloseinaccessible to chemical decomposition. The same problem arises with theenzymatic hydrolysis of cellulose.

As a consequence, existing processes using aqueous acids or enzymes tocatalyze the decomposition of cellulosic compounds by hydrolysis give apoor yield or are very energy consumptive. This renders such processesuneconomic and impractical when, for example, the controllingconsideration is the comparative cost of conventional fuels and anequivalent amount in terms of energy content of ethyl alcohol producedby saccharification and fermentation.

SUMMARY OF INVENTION

The main object of this invention is to provide an efficient andeconomic process for the saccharification of cellulosic material givinga high yield.

While the glucose resulting from a process in accordance with theinvention may be fermented to produce ethyl alcohol that is directlyuseful as a fuel or in an admixture with gasoline to provide a low-costfuel having a high energy content, the ethyl alcohol derived fromcellulosic waste is also valuable as a solvent, an extractant, anintermediate in the synthesis of various organic chemicals or asessential ingredients in many pharmaceuticals.

Also an object of the invention is to provide a method which extractsnot only glucose from cellulosic material but many useful byproducts, sothat virtually all components of the cellulosic stock are extracted inan exploitable form.

Briefly stated, in a saccharification method in accordance with theinvention, raw cellulosic material from any available source isultimately subjected to hydrolysis in a pressurized container maintainedat a temperature level appropriate to an anhydrous liquid hydrogenchloride, the cellulosic material being immersed in the bath and beingconverted thereby into glucose and other reducing substances.Preparatory steps may be included to increase the yield of glucose andother products.

Thus, raw cellulosic material from any available source such asmunicipal waste may be first subjected to steeping in a causticsolution, causing the cellular structure of the raw stock to swell torender it more reactive. Lignins and other components present in thematrix are solubilized by the solution and then washed away so that thecellulosic material is now in the prepared state.

In another or second preparatory step, the prepared cellulosic materialis subjected to viscose processing in which it is treated with carbondisulfide to produce a viscous solution of sodium xanthate in aqueoussodium hydroxide. The aqueous phase is separated from the residue, aviscose intermediate.

In the final step, the viscose intermediate is subjected in apressurized chamber maintained at a temperature level appropriate to ananhydrous liquid hydrogen chloride bath to hydrolysis, the intermediatebeing immersed in the bath and being converted thereby into glucose andother reducing substances.

In practice, raw cellulosic material may be hydrolyzed by anhydrousliquid hydrogen chloride without preparatory steps to provide asignificant yield of glucose. This may be fermented to produce ethylalcohol. In this way, municipal waste and other waste material rich incellulose, instead of being dumped or otherwise disposed of at high costto the community, may be transformed into a valuable fuel.

Because this process eliminates the high percentage of water included inprior techniques using aqueous acids as catalysts or enzyme solutions,the yield of free sub-units is increased to a substantial degree beyondthat heretofore attainable. And because the reaction is highlyexothermic and energetic, and water in bulk is not present, manyintermediate compounds are formed which are exploitable.

DESCRIPTION OF INVENTION

The method in accordance with the invention is applicable to anycellulosic source such as waste wood, pulp, paper, hay, fallen leaves,bagasse, sawdust or any finely-divided plant wastes as well as theeffluents from paper and pulp processing, such as the Kraft or sulfiteprocesses. It is also applicable to municipal wastes, in which case thewaste slurry, after being dewatered in a filter press, is constituted bya mass having a total solids content largely composed of cellulosicmaterials.

The products derived from the method may vary in their relativeproportions, depending on the nature of the starting cellulose source.By way of illustration, we shall apply the method in accordance with theinvention to wood sawdust. Accordingly, the basic nature of this woodwill first be considered.

All wood substances are composed of two chemical materials: lignin and apolysaccharidic system which is termed "holocellulose." Holocellulose iscomposed of cellulose and hemicelluloses, a mixture of pentosans,hexosans, and polyuronides, and in some cases, small amounts of pecticmaterials.

In a wood cell, the outer portion of the primary wall is heavilylignified and the intercellular substance is also mainly lignin, thelignin serving as a matrix in which the cellulosic cells are embedded.Dissolution and removal of the lignin therefore results in separation ofthe wood fibers.

In addition to the cell and wall tissue, which is the fundamentalmaterial of all wood substance, wood contains a variety of othermaterials which are extractable by selected solvents. These extraneouscomponents lie mainly within the cavities of the cells and the surfacesof the cell walls, and include aliphatic hydrocarbons, resin acids, dyesand proteins. Generally, however, the total amount of the extraneouscomponents is only a few percent of the total wood weight.

STEP I (STEEPING)

The sawdust (or other raw cellulosic material to be processed) is firstimmersed in a caustic solution such as sodium or potassium hydroxide toeffect mercerization thereof. Usable for this purpose is a 17 to 20percent solution of sodium hydroxide at a temperature of between 18 to25° C.

During this treatment, the hydroxyl groups of the cellulose reacts withsodium hydroxide to form alkali cellulose. However, the primary functionof this steeping step is to increase the reactivity of cellulose tosubsequent processing by swelling the fibers thereof and also to renderthe lignins and other compounds present in the cellulose matrix soluble.

The sawdust so treated is then washed with steam, solvents or water, orcombinations thereof, to remove therefrom the ligneous substances andother aromatic, polar and nonpolar soluble substances such as rosin. Theremoved compounds are usable commercially as solvents, varnishes, glues,paint constituents, cleaning compounds, etc., and they thereforerepresent valuable byproducts of the preparatory process. What remainsafter these compounds are removed is a prepared cellulosic material.

STEP 2 (VISCOSE PROCESSING)

Viscose is the industrial term for a viscous solution of sodiumcellulose xanthate in an aqueous sodium hydroxide, the solution havingan orange-red color. The principal raw materials for the viscoseprocessing are cellulose, sodium hydroxide, carbon disulfide and water.Industrial processes based on viscose are widely used for themanufacture of rayon and cellulose film or cellophane.

In the second step, the prepared cellulose material derived from stepone is viscose-processed in the manner described in U.S. Pat. Nos.2,855,321 and 2,985,674, or in accordance with any other known viscoseprocess.

Upon completion of viscose processing, the aqueous phase is separatedfrom the solution, leaving a viscose intermediate which is ready to betreated in step three. The liquid so removed is useful as a byproductfor the production of rayon, cellophane and related products.

STEP 3 (HYDROLYSIS)

In this final step, the viscose intermediate derived from step 2 ishydrolyzed by immersing it in a bath of anhydrous liquid hydrogenchloride. Since this cryogenic liquid must be maintained under pressureat a relatively low temperature so that it remains below its boilingpoint, this step is carried out in a pressurized chamber underrelatively low temperature conditions, bearing in mind the knownrelationship between vapor pressure and temperature for hydrogenchloride in its liquid and gaseous phases, as set forth in the MathesonGas Data Book (page 2).

As pointed out previously, raw cellulosic material, instead of goingthrough the above-described preparatory steps, may be directly subjectedto anhydrous liquid hydrogen chloride to provide the desired products.The nature of the raw cellulosic material will determine whetherpreparatory steps are necessary or desirable. For example, disaccharidecellobiose is completely hydrolyzed when immersed in a bath of anhydrousliquid hydrogen chloride. On the other hand, Kraft fibers, uponimmersion in this bath for the same period of time and under conditionssimilar to those used for the hydrolysis of cellobiose, yield onlyapproximately 15 to 20% water-soluble substances, of which about 5% isglucose.

Because hydrolysis with anhydrous liquid hydrogen chloride eliminatesthe bulk of water accompanying hydrolysis of cellulose by aqueous acid,the yield of free sub-units is much higher than has heretofore beenobtainable. The products resulting from the reaction with anhydrousliquid hydrogen chloride starting with raw sawdust are many usefulreducing substances, including glucose, galactose, pentoses,homogentisic acid, ascorbic acid, paraldehyde and lactose.

The sugars and carbohydrates generated by a method in accordance withthe invention may be used directly as foodstuffs or livestock feed, aswell as the starting material for ethyl alcohol production by knownfermentation techniques. The salient advantage of the present methodover prior techniques is that its useful yield is much higher; for itmakes full utilization of virtually all components present in the rawcellulosic stock. Thus the method is economically feasible to produceethyl alcohol as a fuel competitive with existing petroleum-derivedfuels.

IMPROVEMENTS

While the above-described process may be readily carried out on a batchbasis, it is also possible to derive glucose and other useful productsfrom raw cellulosic stock in a continuous process. A continuous processnot only has the advantage of rendering adjustable the control ofreaction conditions but it also reduces the time required to process anyfixed quantity of cellulosic stock. Moreover, it makes it feasible, on acommercial scale, to handle chemical reactions involving highly reactiveor toxic substances, such as anhydrous liquid hydrogen chloride or anyother anhydrous liquid hydrogen halide capable of hydrolyzing cellulosicmaterial.

In a preferred continuous process arrangement in accordance with theinvention, use is made of a reactive chamber in the form of a verticalcolumn containing a bath of anhydrous liquid hydrogen chloride (or otherhydrogen halide), which is the reactive substance. Also in the reactionchamber is a liquid carrier constituted by a non-reactive substancewhose nature is such that it goes into the liquid phase under the sameconditions which liquefy anhydrous hydrogen chloride. The nature of thecarrier must also be such that it is immiscible with the reactive liquidand has a different density therefrom.

The difference in the density of the non-reactive liquid carrier and thereactive liquid is such as to cause one to float over the other.Suitable as a non-reactive carrier for use in conjunction with reactiveanhydrous liquid hydrogen chloride is a straight chain aliphatic such asn-octane. This liquid has a density of about 0.8 gm/ml, as compared toliquid hydrogen chloride whose density is about 1.1 gm/ml. As aconsequence of this difference, the carrier liquid will float on thereactive liquid in the reactor.

Alternatively, one may use a perfluorocarbon liquid as the liquidcarrier with a density usually higher than 1.1 gm/ml, in which case thecarrier liquid would sink to the bottom of the reaction vessel and thereactive liquid would float thereon. The choice of the liquid carrier isdetermined not only on the basis of its physical properties, but alsobecause it is chemically non-reactive with the reactive liquid as wellas with the cellulosic stock to be converted, so that the carrier liquidserves only to transport the stock through the reactor chamber and isindefinitely reusable for this purpose.

In operating the continuous system, the cellulosic stock is introducedinto a stream of carrier liquid and is physically carried thereby intothe reactor without the stock going into solution in the carrier andwithout any chemical reaction therebetween. Because a carrier fluid isbeing handled, pressurizing, pumping, filtering and other processrequirements may be effected using standard hydraulic technology forthis purpose.

Assuming, for purposes of illustration, that the selected liquid carrieris lighter than the reactive liquid, the carrier bearing the cellulosicstock is introduced into the bottom of the reaction chamber so that itwill rise through the bath of the reactive liquid. With a relativelyheavy carrier, it may be advantageous to introduce the carrier into thetop of the reaction chamber so that it then proceeds to sink through acolumn of the reactive liquid.

As the carrier rises (or falls) through the reactive liquid in thereactor, the cellulosic stock borne thereby is exposed to and interactswith the reactive liquid. The transit time of the carrier and the heightof the carrier column in the reactor will determine the time availablefor the reaction between the cellulosic stock and the reactive liquidhydrogen chloride to take place.

Depending upon the respective natures of the reactive liquid, the liquidcarrier, the reactants and the reaction products, layers of reactants orreaction products will develop within the density gradient of thevertical reaction chamber. It becomes readily possible, therefore, tocontinuously tap off the carried treated products from the chamber andthereby continue to make space available for the incoming flow of liquidcarrier bearing raw stock to be treated. The velocity of flow is, ofcourse, governed by reaction time requirements: the lower the velocity,the longer the reaction time.

The exudate from the reaction chamber is fed into suitable separationdevices. Since what is taken out of the reaction chamber is a mix ofcarrier liquid, reactive liquid and treated cellulosic material, andpossibly some yet untreated cellulose, the separation devices must bedesigned to segregate these components.

From the separation devices in which, for example, hydrogen chloride maybe separated by evaporation from the liquid carrier and reliquefied, theliquid carrier and the reactive liquid are fed back for recycling,whereas the treated cellulosic material is discharged, this representingthe useful output of the system. Filtration may be used to separate theuntreated cellulosic stock from the treated material yielded in theoutput, the untreated stock being returned to the carrier stream feedingthe reactor for treatment therein. The recycled liquid carrier serves,of course, to pick up fresh stock for treatment.

It will be appreciated that the pressure requirements for the reactionchamber may be different from those within the carrier lines and thatthis may dictate appropriate pumps and valves utilizing standardhydraulic technology. To increase the yield of hydrolysis products, usemay be made of catalysts in the reaction chamber, such as aluminumtrichloride.

While there has been shown and described a preferred embodiment forsaccharification of cellulose in accordance with the invention, it willbe appreciated that many changes and modifications may be made thereinwithout, however, departing from the essential spirit thereof.

I claim:
 1. A method for converting raw stock that is rich in celluloseinto glucose and other useful products, comprising the steps ofimmersing the stock in a bath of anhydrous liquid hydrogen chloride tohydrolyze the cellulose, and separating the reaction products from thebath to provide said glucose and other products.
 2. A method as setforth in claim 1, further including the step of fermenting said glucoseto produce ethyl alcohol.
 3. A method as set forth in claim 1, wherein acatalyst is added to the reaction chamber to enhance the hydrolysisactivity.
 4. A method as set forth in claim 1, wherein said anhydrousliquid hydrogen chloride bath is contained in a reactor in the form of avertical column into which the raw stock is introduced by means of aliquid carrier stream of a substance whose density differs from that ofthe liquid hydrogen halide and which is non-reactive with said halideand said stock, whereby the stock borne by the carrier passes throughthe bath to produce said reaction products which are withdrawn from thereactor.
 5. A method as set forth in claim 4, wherein the exudatewithdrawn from the reactor is constituted by said reaction products,carrier liquid and liquid hydrogen chloride, these being then separatedfrom each other, the carrier liquid and the liquid hydrogen chloridebeing recycled.
 6. A method as set forth in claim 5, wherein saidcarrier stream is fed into said reactor at the same rate at which theexudate is withdrawn to provide a continuous process.